This disclosure is directed to the use of data obtained by a borehole TV system (BHTV) wherein the data is used to define the dip and strike of strata or beds intercepted by the well borehole. In a broader aspect of the invention, the dip and strike of formations observed on the wall of the borehole by other types of sensors forming an image may be determined. Along a drilled well, the well borehole will intercept various and sundry formations defined by formation interfaces or boundaries, or perhaps fractures, or other indicia of formation position and orientation. In a simplistic way, formations generally are parallel with a dip subject to disturbances involving some geological event or associated fractures in the near vicinity. Alternately, the strata may be approximately horizontal, but the well may not be vertical as in the instance of slant drilling from an offshore platform. A dipmeter has been used heretofore to provide an indication of the fracture, strata or interface dip angle relative to the borehole.
It is now possible to place a centralized BHTV in a uncased well and to rotate the view so that information is obtained from the side wall of the uncased borehole. The output signal of the BHTV forms a strip of film, so to speak, as the BHTV is moved along the borehole and thereby provides an indication of the reflection from the side wall. The film is analyzed to detect fractures, strata, interfaces or other boundaries intercepted by the borehole. The scanned film thus includes an indicia where such formation interfaces, fractures, or changes in structure are intercepted. Dip and strike can be determined.
The film provided by such a BHTV will include an obscured sinusoidal waves in the data. If perchance the fracture or strata or other interface is substantially horizontal to a vertical borehole, it will be appreciated that the sinusoidal waveform will be reduced substantially to a straight line on the film. But, however, when a sinusoidal waveform is observed in the film data, it is indicative of a formation interface or some other aspect intercepted by the borehole which might be described in spatial orientation, i.e., by the dip and strike. The BHTV provides the output film where color intensity (often encoded in a gray code) will be involved in such a sinusoidal waveform. This film has contrasts in color, texture, etc. occurring where an interface or fracture is intercepted by the well borehole. Assume, for instance, that a sharply defined interface is intercepted by the borehole. In that event, the film should present the color contrast or texture contrast on the film as a sinusoidal waveform. Given this waveform in the data on the film, the present disclosure is directed to a method of processing the output data of a BHTV so that the data can be converted into information regarding the interface. The interface is primarily located by determining the dip and strike angles of the interface. This data can be obtained by measuring the amplitude and phase of the sinusoidal image in the film. The terms dip and strike have well known definitions when applied to interfaces, fractures, strata, etc.
With the advent of borehole imaging, a primary concern of the log analyst is to obtain dip and strike information from sinusoidal patterns found in these images. In the past, these solutions for the patterns were calculated manually, taking into account the effects of decentralization and borehole shape, see D. T. Georgi, "Geometrical Aspects of Borehole Televiewer Images", SPWLA 26th Annual Logging Symposium, Paper O, June 1985. More recently, these patterns are manually identified on workstations and fitted to sinusoids via the computer. See R. A. Plumb, S. M. Luthi, "Analysis of Borehole Images and Their Application to Geologic Modeling of an Eolian Reservoir" SPE Formation Evaluation Symposium, December 1989, pp. 505-514, J. K. Faraguna, D. M. Chase, M. G. Schmidt, "An Improved Borehole Televiewer System: Image Acquisition, Analysis and Integration" SPWLA 30th Annual Logging Symposium, Paper UU, June 1989 and Lyel, et al. "Method for Logging the Characteristics of Materials Forming the Borehole of a Wall" U.S. Pat. No. 4,780,857, Oct. 25, 1988. The amplitude and phase of these sinusoids, properly combined with caliper data, provide the dip and strike angles of relevant formation features.
A representative data collection device is known as the CAST tool. The CAST tool (Circumferential Acoustic Scanning Tool) is an enhanced version of the BHTV, is described on the principles of Zemaneck which produces two types of data; namely, amplitude and time of flight. The amplitude data measure the peak acoustic reflection of the borehole wall, and the time of flight determines the distance from the tool to the borehole wall. The tool acquires this information from a rotating sonic transducer located at the bottom of the sonde. The transducer is fired at a fixed but selectable rate ranging from 100 to 500 times per rotation. At each firing the tool measures the strongest echo (amplitude) and the time elapsed between the firing and the detection of its echo (time of flight). The tool typically rotates at different rates up to 120 times per vertical foot. The CAST tool may be used as described, or alternate forms of BHTV tools may be used. In any event, the tool forms an image or collection of images on a data record for use in accordance with the teachings of this disclosure. Other BHTV systems beside the CAST tool may be used.
The acquired and recorded information may be presented either as vertical or horizontal waveforms, or as two dimensional images. In the two-dimensional case, the horizontal dimension is produced by "cutting" the borehole along the north azimuth (or some other selected point) with the sidewall unrolling onto a flat film surface while the vertical dimension of the image corresponds to the depth of the tool. The value of the data at a particular depth and azimuthal coordinate is represented by an intensity level (amplitude). While the illustrated and preferred mode of data collection is a BHTV system, the present method also operates well with alternate methods of gathering data regarding the borehole wall. Where a data film is mentioned, the data can readily be on a film or recorded in some memory.
The process set forth hereinbelow contemplates operation of a BHTV in an uncased well borehole forming an output signal which is normally presented as pixels of color or gray tone in a memory, one organization thereof representing data on a strip of film. The data (in digital form) is first normalized relative to certain standards on a typical gray scale. The image is then enhanced in digital form primarily by digitally removing background noise. The data representing the scanned pixels in digital form encodes formation boundaries which are represented in a more contrasted fashion, e.g., black data on a white background, but still in a digital form. At this juncture, the enhanced and modified data in digital form will better represent formation boundaries, fractures, etc. in a sinusoidal waveform. The process continues by applying a Hough Transform to the data so that data points on a curve are, mathematically speaking, transformed into parameter space where the dip and strike angles are dimensions within parameter space. Accordingly, a single point (encoded in digital form) of a sinusoidal data curve in the binary data represents an entire curve in parameter space. By the choice of multiple points from a curve, different curves in parameter space can be presented. The loci of multiple points should cross or intercept at a common parameter space intercept. Since noise is always present, the intercept of the several loci may not be quite precise, but there will be a clear indication of the intercept. This enables determination of dip and strike angles which can then be output to identify the boundary.
Consider one example of data from an operative BHTV system. The data (after noise reduction) may have the form of a mix of binary ones in a field of binary zeroes. A simplistic two value, one bit system is assumed for tutorial purposes; in actuality, a multiple bit data representation is used so that the multiple values range between black and white and include various gray tones. The number of tones is normally a multiple of two, for instance, a total of thirty-two or sixty-four tones may be used. In this regard, some dividing line is defined between black and white so that the data can be processed through further steps. Assume that a stretch of borehole has been encoded by 300 revolutions of the BHTV and that each revolution is divided into 360 data points. This defines a block of data which is 108,000 pixels. Those which show a data entry above a selected value of color or tone are encoded as a binary one in two value system while all the remaining pixels are encoded as zeroes. Each data entry represents a possible point on a sinusoidal curve, therebeing the possibility of several or perhaps no interfaces in the data. Each data point (representing a pixel) is transformed. After the transform, those data common to an interface will collectively indicate that particular interface with strike and dip. By contrast, data not resultant from an interface will not collectively indicate an interface and will therefore be easily discarded.
The images contain information about borehole events, shape, and formation properties. In general, the borehole events are classified as either vertical or horizontal features. Vertical events correspond to vertical fractures, pad marks, etc. Horizontal events, on the other hand, correspond to fractures intersecting the borehole at oblique angles, to erosions of an interface between bedding planes, to dipping formations, etc. The horizontal events are the only events of interest in the ensuing discussions.